Area Forecast Discussion
Issued by NWS Honolulu, HI

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645
FXHW60 PHFO 172000
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Tue Jan 17 2017

.SYNOPSIS...
An increase in trade winds is expected through Wednesday, with
locally breezy and mostly dry conditions continuing into Thursday.
Winds will diminish Thursday night and Friday, becoming light and
variable as a front approaches. The front will move through the
island chain beginning Friday night, bringing a few showers to
windward areas. Trade winds are expected to become quite strong this
weekend after the front passes and a strong high passes north of the
area.

&&

.DISCUSSION...
In summary, an E-W oriented surface ridge just N of Kauai will move
N over the next 12 to 24 hours as a fast-moving high pressure cell
passes N of the islands. In response, E trade winds are expected to
increase and become locally breezy over Maui county and the Big
Island, with moderate E to ESE trade winds expected over Oahu and
Kauai through Thursday. Winds will diminish Thursday night and
Friday, becoming light and variable as the ridge dissipates. A new
high will build far NW of the islands Friday night and Saturday,
pushing a front over the islands, while also bringing a significant
increase in NE trade winds this weekend. The island atmosphere will
be very stable, especially the next couple of days, and mostly dry
weather will prevail until the front brings a few showers that will
primarily moisten windward areas.

The island atmosphere is as stable as it gets this morning, with the
overnight sounding from PHLI indicating that the base of a very
strong subsidence inversion is near 3 kft, while the PHTO sounding
indicated an inversion height near 5 kft. Visible satellite imagery
centered over the Hawaiian islands shows a difference in cloud
character from NW to SE across the area, with broken to overcast
closed-cell stable stratus prevailing over most areas, except near
and E of the Big Island, where open-celled showery cumulus clouds
are seen. Thus the short-term expectation is that the windward side
of the Big Island will see a few showers riding in on the trade wind
flow, with nary a shower elsewhere. The stable low clouds are
expected to diminish through the morning as they burn off.

A nearby strong mid-level ridge is responsible for the stable island
atmosphere, and is also buffeting the Big Island summits with strong
E to SE winds. A Wind Advisory is in effect today, and may need to
be extended, as latest guidance indicates free-atmosphere winds will
be strong enough to support advisory-level winds over the summits
into Thursday.

Trade winds will increase the next 12 to 24 hours as a bubble high
passes N of the islands and the ridge that is now just N of Kauai
lifts N. The ridge will still remain fairly close to Kauai, so E to
SE winds will peak at moderate levels on Kauai and Oahu tonight and
Wednesday, with locally breezy trades expected across Maui County
and the Big Island. Little in the way of rainfall is expected
outside of a few showers over windward portions of Maui and the Big
Island. With the ridge dissipating, Thursday night and Friday will
be dominated by land and sea breezes, with clear skies at night and
morning with afternoon interior cloudiness resulting in little
precipitation.

A significant trade wind event is becoming increasingly likely this
weekend as a strong high passes N of the islands and drives a
frontal boundary over the islands from the N. Differences still
exist in the forecast guidance but the general expectation is for a
front to move over the islands beginning Friday night on Kauai and
moving to the Big Island by Saturday night. GFS indicates the
frontal moisture will move SE of the Big Island Sunday before the
associated moisture moves back in on the trade wind flow on Monday.
Little to no confidence in this aspect of the forecast at this time.

Winds will increase rapidly as the front moves through, with latest
GFS and ECMWF guidance depicting a period of 25-30 kt sustained
winds over the area this weekend. If this verifies, then a statewide
Wind Advisory would be warranted. A shallow and relatively narrow
cloud and shower band will accompany the front, bringing a few
showers that will primarily affect windward areas. The wind-
supporting high will be moving rapidly E, so N to NE winds on
Saturday will veer to the E by Sunday, and weaken rapidly on Monday.
However, another high is forecast to build NW of the area Tuesday,
pushing another front over the islands and bringing another increase
in trade wind speeds.

&&

.AVIATION...
The low-level trade wind flow, which is slightly stronger near Maui
County and the Big Island compared with Kauai and Oahu, will
continue to transport low clouds onto the windward facing slopes and
coasts of the islands today. The trades may carry a few showers onto
windward Big Island as well today. Brief periods of MVFR CIGS are
possible in the passing low clouds and showers over windward
sections. Otherwise, VFR conditions are expected to prevail.

There are currently no AIRMETS in effect late this morning. As the
trade winds continue to strengthen today, there is a possibility
that AIRMET TANGO may be required for TEMPO moderate low-level
turbulence leeward of the mountains on some islands this afternoon
or tonight.

&&

.MARINE...
Easterly trade winds will build later today and tonight as an area
of stronger high pressure races by to the north of the islands.
An ASCAT pass from last evening showed no Small Craft Advisory (SCA)
strength (25 kt) winds over local waters, but due to the expected
increase in the trade winds, an SCA remains posted for waters around
the Big Island and Maui County through Wednesday afternoon. Trade
winds will gradually diminish on Thursday and Friday as the area of
high pressure to the north is eroded by an approaching front. The
front is expected to produce strong NE winds and large, short-period
seas during the weekend, leading to widespread SCA conditions and
the possibility of local gale-force winds.

Moderate WNW swell will continue today. The next large WNW swell
will build late tonight, peak late Wednesday, then drop to moderate
levels by Thursday night. This swell is expected to be similar to
the last large WNW swell from a couple of days ago. As a result,
expect warning level surf on Wednesday and Wednesday night for most
N and W facing shores of the smaller islands and seas above the 10
foot SCA threshold over most Hawaiian waters. The Kona coast of the
Big Island will also experience surf near or at the warning level
late Wednesday night and early Thursday. A front expected to move
down the island chain during the weekend will produce large, short-
period seas from the NE, and a High Surf Advisory will likely be
needed for E facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Big Island Summits.

Small Craft Advisory until 6 PM HST Wednesday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST
Wednesday for Maui County Windward Waters-Maalaea Bay-Pailolo
Channel-Big Island Windward Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Foster
MARINE...Wroe/Stall



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